Stars of the sky, like stars of the silver screen, hide their age well. An ancient star can often pass for a much younger one. And the question of age becomes pretty important as astronomers seek out potentially habitable planets orbiting distant stars.

“Is it perhaps 4.6 billion years old? Well all of a sudden we know we could have a planet teeming with complex and intelligent life.” But a star's age is not always obvious. “Well, stars do not have birth certificates.”

One feature does change with time. Older stars tend to rotate more slowly. “And so we can use the spin rate, the rotation rate of a star, as a clock to measure its age.” But someone has to calibrate that clock. So Meibom and his colleagues are measuring the rotation of stars whose ages they already know. If they can pin down the relationship between age and rotation, a star's age will be much easier to measure. No birth certificate required.